Charge coupled device (CCD) and complementary metal oxide semiconductor (CMOS) are optical semiconductors that are commonly used as a solid-state image sensing device today. The CCD has been mainly used in video cameras for capturing high-definition images, but the accelerating use of data processing for image has recently broaden the scope of its usage rapidly. Particularly, they are commonly included in digital still cameras or cell phones to convert high-definition images into electronic information data. The CMOS, also called “complementary metal oxide semiconductor”, has advantages as follows. That is, it can be smaller and consumes as little as about one-fifth the power in comparison with CCD. Further, it can be produced by using a microprocessor production process, which leads to less cost for facility and lower production cost. Thus, it is more frequently included in image input devices, such as cell phones or small personal computers.
The solid-state image sensing device is disposed in the semiconductor package made of a ceramic or metal material, such as alumina, or a plastic material, and hermetically sealed by bonding a sheet type cover glass which functions as a light transmissive window with an adhesive consisting of any kind of organic resin or low melting point glass.
The cover glass for semiconductor package is required to have a low emission of alpha-ray, because high emission amount of alpha-ray from the cover glass results in soft errors. The emission of alpha-ray from the glass results from the radioisotopes, such as of uranium (U) or thorium (Th), contained as impurities in the glass. Thus, during the glass production, measures such as using high-purity raw materials, or using refractories (e.g., alumina electrocast refractories, quartz refractories, platinum) with low content of radioisotopes for the inner wall of the melting furnace are taken.
Recently, along with the spread of digital cameras or cell phones, the demand for lightweight and compact high-pixel imaging systems and space-saving of the members are increasing. Thus, the package materials are becoming smaller and thinner. Further, plastic packages are attracting attention with a view to making the whole member lightweight.
Such being the case, the patent document 1 discloses a cover glass for semiconductor package having a thermal expansion coefficient conformable to plastic packages and a low emission amount of alpha-ray.